The present invention relates to a method for optimising print images output by color printers onto substrate surfaces, in particular onto non-white substrate surfaces and for optimising the printing ink quantities used, an image motif being processed by means of a computer-aided image processing system to form an image original which is ready to output. The invention also relates to packaging films printed by the method according to the invention.
Image motifs, with which a substrate surface is to be printed, are acquired or created by means of computer-aided image processing systems and brought into image originals which are ready to output. The processing of the image data into image originals which are ready to output takes place with the aid of appropriate image processing software, such as for example PageMaker(copyright), Quark Xpress(copyright), Barco Packedge(copyright) or Macromedia Freehand(copyright) on DTP systems, wherein xe2x80x9cDTPxe2x80x9d stands for xe2x80x9cDesktop Publishingxe2x80x9d. DTP is a current designation for creating print publications by means of computers. The image data are displayed on a screen according to the principle of additive color mixing, for example in the known RGB format (R=red, G=green, B=blue).
The image original ready to output is then passed to an image output system, converted into a format which can be read by an image output system and printed by a color printer, wherein when printing a non-white substrate surface, a white underprint is executed before the actual print of the image motif. If the printing is a counter-print, white is applied as the last color, in other words after printing the actual image motif, as an overprint on the print image.
As the mode of operation of color printers, such as for example color printing systems operating by the electrophotographic method, is based on the principle of subtractive color mixing, the image data are converted into a subtractive color format prior to transfer to the image output system. The known CMY color space is generally used for this purpose, comprising the three primary color planes cyan (C), magenta (M) and yellow (Y). Cyan corresponds approximately to a blue-green and magenta approximately to a purple. The printing systems here use a cyan, magenta and yellow printing ink, from which further colors can be produced, wherein the primary printing colors act as color filters. Light which falls through a C, M or Y primary printing color, is absorbed or filtered in certain spectral ranges by the printing ink, so only light in a limited spectral range is reflected by the printing ink, and perceived by the human eye as the color of the toner. Theoretically, black can be produced by the ideal mixing of the primary printing colors C, M, and Y, as now all the light is absorbed or filtered. However, in practice a particularly deep and strong black cannot be produced by mixing the primary printing color, so apart from the CMY primary printing color a black (K) printing ink completely absorbing the light is used for black portions and grey levels. The color space supplemented by black is designated the CMYK color space.
In digital image processing systems, the image original is divided into individual image points, also called pixels. A respective value for each of the four primary printing colors is allocated to each image point, for example by using the CMYK printing space. This value represents the so-called color density. For each image point various mixed colors can be shown with the four color planes and the color density values allocated to them.
The color density, also called color covering, is a standardised variable for the applied quantity of printing ink. The color densities FC for C (cyan), FM for M (magenta), FY for Y (yellow) and FK for K (black) are in a defined range of 0 to 1 or 0 to 100%, wherein 1 represents a maximum application of the corresponding printing ink and 0 no application of the corresponding printing ink. The sum of the individual applied color densities are called the total color density. The application of the maximum color density FC, FM and FY of the three primary colors CMY therefore produces the high total color density or total color covering of 3 or 300%.
The image data of the image original are either acquired in the form of raster or vector data. Accordingly, the image original can be present in a bitmap or vector graphics data file format. Standardised vector graphics data file formats are, for example PostScript (PS) which inter alia includes Encapsulated Postscripts (EPS) or Portable Document Format (PDF). A standardised bitmap or raster graphics format is, for example Tagged Image File Format (TIFF).
Generally the image originals which are ready to output are placed in PostScript data files, these data files, apart from the actual image data, containing further information necessary for further processing of the image data, for example with reference to formatting and instructions such as, for example, control instructions to the image output system. Postscript data files may also contain inter alia image objects present in a bitmap format. The image original can therefore be, for example, an object embedded in the PostScript data file and present in a raster graphics format, for example TIFF format.
Digital image output systems generally contain a xe2x80x9cRaster Image Processorxe2x80x9d (RIP) and a printing unit. The raster image processor (RIP) determines from the image original supplied, for example in a PostScript data file, the size, quantity and position of the image points (pixels) and converts these into a format which can be interpreted by a printing unit. The image data converted into printing instructions are converted in the printing unit into a color print.
The current image processing systems are designed for printing white substrate surfaces, in particular white paper. The white substrate surface is thus generally included in the coloring process in image processing. White is for example generated by the allocation of the total color density CMYK=0, in other words image points with the corresponding zero value contain no color application. Apart from showing white surfaces, white is also necessary for showing the color brightness. The color brightness can be determined, on the one hand, by varying color densities and, on the other hand, by a raster display of the image points.
To obtain the same or a comparable color impression when printing colored, translucent or transparent substrates, as is produced during printing of a white underlay, the substrate surface provided for printing is therefore underprinted with white prior to the actual application of the print image. Underprint means that the white printing is located under the actual print image directly on the substrate surface. In the case of counter-printing on a transparent or translucent substrate with a colored, translucent or transparent, in particular non-white, substrate resting on the counter-print, a white overprint is applied for the above-mentioned reasons directly on the print image.
However, in image regions with an adequately large total color density, i.e. in image regions in which the white substrate surface is not visible or does not shine through owing to a large color application, a white underprint is not necessary. The dark color tones, i.e. the places with a high total color density, are frequently faded or they even appear unsaturated owing to the white underprint, so in order to achieve deep colors the overlying total color density has to be additionally increased.
Because of the surface-covering white underprint or overprint and the print image arranged thereabove, a very high color application is also, moreover, often achieved and this can impair the melting of the toner, for example, in the electrophotographic printing method and the image quality.
Basically, the image processing can be oriented to the specific color properties of the substrate surfaces to be printed. However, this requires adaptation of the corresponding image processing systems, in particular image processing software, connected with high expenditure.
The object of the invention is therefore to provide a method for creating an image original which is ready for printing for visually colored, translucent, transparent, specular or metallic appearing substrate surfaces or substrates and for the printing thereof, wherein the drawbacks resulting because of the above-described white underprint or overprint are to be eliminated, without expensive adaptation of the image processing software being necessary.
According to the invention, the object is achieved by a method for optimizing colored images emitted by a color printer on the non-white surfaces of substrates and for optimizing the amounts of printing ink used, wherein an image motif is processed by a computer-assisted image processing system in order to form a master copy which is ready for output. The method determines, for each pixel, whether and with what color density, a white underprint can be applied to a corresponding pixel, using an algorithm based on overall color density SF. The surface of the substrate is thus only underprinted with white on the pixels of the master copy where the overall color density is lacking or low.